In order to reduce the amount of harmonics current in power supply apparatuses, in other words, in order to improve the power factor thereof, it is considered to use a so-called active harmonics filter or a so-called capacitance-less converter, which does not have a smoothing capacitor at an input side thereof. In such an active harmonics filter or a capacitance-less converter, a smoothing capacitor is not provided after the rectifying section, and a rectified line current is directly switched by a switching element, so that a necessary and sufficient power factor can be obtained and the construction thereof is very simple. However, they also have disadvantages such that a significant amount of output ripple occurs or a holding time is not apt to be sufficiently kept for an instant power failure of about 0.02 sec. Therefore, a so-called two-stage converter type power supply apparatus is used, in which a DC-DC converter is provided after the active harmonics filter. In such an apparatus, an electric current must go through two-stage converters, so that a switching loss is caused twice and then the efficiency becomes low.
On the other hand, some single switching element type converters are suggested in which a holding time can be kept and the output ripple can be reduced so as to overcome the disadvantage of the conventional capacitance-less converter type apparatus while a power factor there is also improved. However, almost all of these apparatuses are designed as an energy circulating type converter or a resonance type converter; therefore an electric current is circulated at the primary side of the apparatus. In addition to this, when an electric current goes through diodes or inductors a loss of electric current is caused so that efficiency becomes low.
Further, in case of applying a forward type circuit to a capacitance-less converter having a single switching element, it is not so easy to combine a forward type circuit with the capacitance-less converter. Because dealing with a reset current is difficult and a more complicated contrivance is required to realize the combination.
The object of the present invention is to provide a single-stage converter type power supply apparatus where the energy loss, caused by two-stage switching operations in a two-stage converter type power supply apparatus, is reduced to improve the efficiency of the apparatus.
Another object of the present invention is to provide a single stage converter type power supply apparatus where a power factor is improved without reducing the efficiency, and the holding time can be kept long and also the ripple characteristic can be improved.
As stated above, if it is tried to keep a sufficient holding time and to improve a ripple characteristic while improving a power factor in a single stage converter type power supply apparatus, one cannot avoid making to make the switching loss large. There are two types of circuit system for a single stage converter type switching power supply system, one of which is the flyback type circuit system and the other one is a forward type. The former has an advantage that is easy to control while having a disadvantage in that the efficiency is somewhat low and the size of the transformer must be large. On the other hand, the latter has an advantage in that the efficiency is high and the size of the transformer can be made smaller; however, it is required to give a reset current in order to prevent a saturation of the cores and therefore it is difficult to control the system because of the limitation for dealing with the reset current. For this reason, almost all of the capacitance-less converters use the flyback type circuit system.
In addition to this, capacitance-less converters have disadvantages in that a sufficient holding time cannot be obtained under a power failure condition because energy cannot be stored at the primary side of a transformer and a great amount of output ripple occurs.
The present invention has solved these problems by means explained below.